Comet C/2013 A1 (Siding Spring)

J. Crovisier, P. Colom, N. Biver, and D. Bockelee-Morvan, LESIA, Observatoire de Paris, report the detection of the 18-cm OH lines in comet C/2013 A1 with the Nancay radio telescope. This detection was favored by the enhanced continuum background, as the comet was close to the Galactic plane. Using the Galactic continuum from the 1420-MHz maps of Reich et al. (2001, A.Ap. 376, 881), Crovisier et al. derive OH production rates of 1.3 (+/- 0.3) x 10**28 molecules/s (average of data from Oct. 5-8) and 3.3 (+/- 0.4) x 10**28 molecules/s (average of data from Oct. 11-13). This latter value is possibly overestimated due to uncertain Galactic background. S. Nakano, Sumoto, Japan, writes that his latest orbital elements (from 1293 astrometric observations spanning 2012 Oct. 4-2014 Oct. 12; mean residual 0".53), given below, show that comet C/2013 A1 will make a close approach to Mars on Oct. 19.77 TT, at a distance of 0.000942 AU. He finds original 1/a = +0.000025 and future 1/a = +0.000104 (+/- 0.000001). Epoch = 2014 Oct. 30.0 TT T = 2014 Oct. 25.30223 TT Peri. = 2.42312 e = 1.0004531 Node = 300.97645 2000.0 q = 1.3987148 AU Incl. = 129.04284 Visual total-magnitude and coma-diameter estimates: July 2.82 UT, 11.9, 1' (M. Mattiazzo, Swan Hill, Victoria, Australia, 20-cm reflector); 21.34, 11.5, 1' (A. Amorim, Florianopolis, Brazil, 0.18-m reflector); Aug. 9.34, 10.1, 3' (M. Goiato, Aracatuba, Brazil, 0.22-m reflector); Sept. 3.75, 9.6, 2'.5 (P. Camilleri, Warners Bay, Australia, 0.40-m reflector); 22.42, 9.7, 2' (Camilleri, 0.20-m reflector); Oct. 1.44, 10.6, 2' (Camilleri, 0.40-m reflector); 15.42, 11.4, 1'.7 (K. Yoshimoto, Yamaguchi, Japan, 0.25-m reflector).